# std::valarray<T>::apply

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< cpp‎ | numeric‎ | valarray

valarray<T> apply( T func(T) ) const; |  |
valarray<T> apply( T func(const T&) ) const; |  |

Returns a new valarray of the same size with values which are acquired by applying function `func` to the previous values of the elements.

### Parameters

func | - | function to apply to the values

### Return value

The resulting valarray with values acquired by applying function `func`.

### Notes

The function can be implemented with the return type different from std::valarray. In this case, the replacement type has the following properties:

  - All const member functions of std::valarray are provided.
  - std::valarray, std::slice_array, std::gslice_array, std::mask_array and std::indirect_array can be constructed from the replacement type.
  - For every function taking a const std::valarray<T>& except `begin()` and `end()`(since C++11), identical functions taking the replacement types shall be added;
  - For every function taking two const std::valarray<T>& arguments, identical functions taking every combination of const std::valarray<T>& and replacement types shall be added.
  - The return type does not add more than two levels of template nesting over the most deeply-nested argument type.

### Possible implementation

Following straightforward implementations can be replaced by expression templates for a higher efficiency.

    template<class T>
    valarray<T> valarray<T>::apply(T func(T)) const
    {
        valarray<T> other = *this;
        for (T& i : other)
            i = func(i);
        return other;
    }

    template<class T>
    valarray<T> valarray<T>::apply(T func(const T&)) const
    {
        valarray<T> other = *this;
        for (T& i : other)
            i = func(i);
        return other;
    }

### Example

Calculates and prints the first 10 factorials.

Run this code

    #include <cmath>
    #include <iostream>
    #include <valarray>

    int main()
    {
        std::valarray<int> v = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        v = v.apply([](int n) -> int
                    {
                        return std::round(std::tgamma(n + 1));
                    });
        for (auto n : v)
            std::cout << n << ' ';
        std::cout << '\n';
    }

Output:

    1 2 6 24 120 720 5040 40320 362880 3628800

### See also

for_each | applies a function to a range of elements (function template)
ranges::for_each (C++20) | applies a function to a range of elements (algorithm function object)

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